The DFM uses the ATtiny’s internal 16 bit timer, which has the convenient property of being able to be driven by an external clock. The frequency to be measured drives the timer, and the time it returns is compared to the system clock. It’s not the finest of frequency counters, depending as it does on the ATtiny’s clock rather than a calibrated crystal reference, but it does the job.

The results are shown in the video below, and all the code has been posted in his GitHub repository. We can see that there is the basis of a handy little instrument in this circuit, though with the price of cheap multimeters being so low even a circuit this minimal would struggle to compete on cost.